BACKGROUND: The all vanadium redox flow battery (VRFB) has become the most common type of rfb, however, it is essential to improve our understanding of the importance of key operational variables, including electrode materials, electrolyte flow rate, current density and temperature, on the cell efficiency together with improved methods for cell monitoring. RESULTS: Several carbon felts were characterized for their electrode activity. A commercially supplied electrolyte of unknown composition was analysed and was shown to contain a VO2+: V3+ ratio of 4.8:1 (total vanadium species = 1.5 mol dm(-3)) in 4 mol dm(-3) H2SO4. A battery (100 cm(2)) was assembled using three-dimensional carbon felts and planar carbon feeders as the electrodes with a Nafion (R) 115 proton exchange membrane. Performance was examined using electrolytes of varying vanadium ion concentration at volumetric flow rating from 0.5-3 mL min(1). A suitable volumetric flow rate of electrolyte through each half-cell was found to be in the range 1.5-2.0 mL min(-1), corresponding to a mean linear electrolyte velocity of 1.0-10.1 cm s(-1) through the carbon felt electrode. At a constant current charge and discharge current density of 100 mA cm(-2) the typical voltage efficiencies were 65%. Charge-discharge curves were then simulated using a detailed physical model, which generated good quantitative agreement with experimental cell performance. CONCLUSIONS: This study has quantified the effect of key process variables on cell efficiency. A mathematical model has been used successfully to describe charge-discharge performance and the use of open-circuit cell voltage has been shown to provide a simple and useful means of cell monitoring. (C) 2012 Society of Chemical Industry